Science in the Kitchen: Making Yogurt Ice Cream

by Sai Pathmanathan, PhD
Eva Baughman

As parents and educators, we want the younger generation to learn useful life skills such as cooking. We want them to eat healthy foods in the face of temptations like candy bars, cake pops, and ice cream. The World Health Organization estimates that by 2025, 70 million children globally will be overweight and at risk for diabetes, heart disease, osteoarthritis, and stroke.

I work in informal science education. Many of my workshops involve a food element, whether experimenting with it (tasting, smelling, testing) or creating something edible (cooking). And having worked on children’s health projects, I feel a responsibility to make sure my outreach workshops promote healthy eating behavior – encouraging snacks that are low in sugar, low in fat, and that include fruit and vegetables – while still being enjoyable.

The following recipe-activity produces a healthy version of homemade ice cream: frozen yogurt. The yogurt is low-fat and the only sugar content is from natural sugars in the fruit. It’s easy and fun for any age, from small children to adults.

The best recipes are the ones that don’t require fancy equipment. The same applies to the best science activities. Here, you won’t need state-of-the-art ice cream or yogurt makers. A few simple items from your kitchen will do.

Ingredients

You will need (for each child):

• Small or quart-size ziplock bag
• 118 ml (1/2 cup) plain, low fat yogurt
• A selection of fruit (soft fruits such as berries work best)
• Large or 3.7-liter (1 gallon) ziplock bag (or a mixing bowl)
• 473 ml (2 cups or 10-12 ice cubes) crushed ice
• 59 ml (1/4 cup) salt (table, rock or Kosher)
• Kitchen towel (optional)
• Spoon (for tasting the yogurt)

Preparation

1.  In the small bag combine the yogurt and fruit. Seal the bag well (pressing out most of the air inside) and squeeze the yogurt and fruit together so that they mix well.

2. In the large bag (or mixing bowl) combine the ice and salt.

3.  Now the freezing can begin! Put the small bag of yogurt mix into the large bag, and seal the large bag.

4.  Shake and move the ice around the yogurt bag, so that the yogurt in the smaller bag starts to feel solid. This should take 10 to 15 minutes. (If you’re using a mixing bowl, move and swirl the sealed small bag around in the ice-salt mix in the bowl, holding onto the top of the bag.)

5.  Remove the small bag from the ice-salt mix. Pat the bag dry on a kitchen towel to make sure no salt gets into your yogurt (otherwise you’ll have a very salty dessert).

6.  Using a spoon, scoop some of the frozen yogurt from the small bag and have a taste!

What happened?
The yogurt became solid, by freezing, but you’ll also notice that the ice melted quite quickly. This is why salt is sprinkled on icy roads – it helps the snow and ice to melt. Salt lowers the freezing point of ice. The ice takes in heat energy from its surroundings. In our experiment, the ice is taking heat from the yogurt around it, so the yogurt gets colder and colder until it’s frozen.

For an even quicker snack, you can use this method for freezing store-bought sugar-free, low fat fruit yogurt. Or why not try experimenting with milk, smoothies and fruit juices?

Adapting the activity
If you’re making frozen yogurt at home with your children, it can be made in bowls, avoiding the use of plastic bags. Combine the yogurt and fruit in a small metal bowl, and place this in a larger bowl containing the ice and salt. The bowl should rest nicely on the ice, but make sure the yogurt bowl doesn’t gradually sink in as the ice melts. Slowly stir the yogurt with a spoon.

If you’d like to run this activity with a group, in school or afterschool, and use plastic bags, they can be rinsed out, dried, and re-used for other purposes. If you have only a few children taking part, and a large basin for them to easily stand around, this can be used as the container for the ice and salt, and they can swirl their yogurt bags around inside the basin. This activity is safe and suitable for any age child, but make sure you check ahead of time that no one has allergies to any of the ingredients.

The recipe can be multiplied, but the more yogurt you have in each bag or small bowl, the longer it will take to freeze — and you may notice the need for more ice and salt, as the ice will melt very quickly.

My experiences
I run this activity in afterschool science clubs, with children aged 7 to 11 years old, sometimes joined by younger siblings. The youngest children may need a little assistance getting the yogurt and fruit into their bags and sealing the bags securely.

Some children aren’t allergic to yogurt or fruit, but will moan, “I don’t like yogurt.” Some of them have never tried yogurt, let alone frozen yogurt. But by getting them involved in the process, showing them that food preparation is essentially all science, they become eager to taste their creation and are converted. All I hear then is, “That’s delicious. Yum!” Or several weeks later, “Hey, Sai, do you remember when we made frozen yogurt? Can we do that again?”

There is always one child who forgets to seal the yogurt bag, so some salt from the outside will creep in. “Why does my frozen yogurt taste salty?” I ask them to think what the answer might be, and then watch as they start giggling when the light-bulb moment occurs. One of my students liked the salty yogurt. He said it reminded him of the Indian drink lassi.

I believe in showing children through science just how easy it is to make foods that are good for their health, hoping that these little nuggets of information might influence them throughout their lives.